FIELD OF THE INVENTION
Cables are durable industrial goods and must therefore meet higher demands with respect to operational reliability. Damage caused by ingress of water can be avoided by a cable insulation making the cables watertight. In "Drahtwelt", number 5/1992, several methods are described to provide power cables, communication cables, and optical fiber cables with a watertight insulation in the longitudinal direction. The main emphasis is concentrated on discussing swelling powders and swelling nonwovens based on polyacrylate which are incorporated into the cable construction. Superabsorbent polymers have been known for a long time; they are commercially available in the form of powders, for example, under the tradenames FAVOR.RTM. or CABLOC.RTM.. However, powder processing involves heavy technical expenditure, and--as soon as abrasion of the product results in respirable, extremely fine dust portions during processing and production--it is critical from the aspect of industrial medicine and should therefore be avoided. The alternative of polymerizing a monomer solution on the prefabricated surfaces or threads is reserved for chemical works because of the required protective atmosphere and the safety regulations to be observed; for this reason, it cannot be transferred to plants of other industries, for example, the printing, cable, or textile industry. Moreover, a highest possible solids content and rapid swelling of the product should be aimed at to seal and limit water invasion into the cable as quickly as possible.
EP 188 091 seems to solve this problem. It describes that the aqueous solution of an uncrosslinked prepolymer is applied on a nonwoven by padding. A relatively high molecular weight of the prepolymer is chosen to obtain the correct viscosity for padding. The resultant stringing leads to the fact that all fibers are covered with the prepolymer solution. For this reason a very rigid, inflexible nonwoven is obtained after cross-linking, and this is very unfavorable for technical further processing, for example, rolling for storage or transport purposes.
EP 269 778 describes a swellable cable insulating tape made of a nonwoven and having superabsorber particles applied thereon. These superabsorbent particles swell on contact with water and therefore prevent propagation of water along the longitudinal axis of the cable. The problem of dust formation is not solved.
EP 271 171 describes a tape for the insulation of cables which comprises microcapsules expanding at different temperatures and thereby causing the seal of the cable. Expansion of the microcapsules takes place already during the cable production, the manner the cable is sealed against leakage in operating state is not disclosed.
EP 357 474 describes the spraying of low-viscous, aqueous solutions of uncrosslinked polyacrylic acids on sheet-like fabrics and the subsequent thermal cross-linkage to form water-swellable gels by means of simultaneously applied cross-linkers. The problem of the proposed method is that spraying results in an irregular polymer distribution on the surface of the sheet material, and that the other forms of application briefly mentioned in this publication, such as printing or doctor-blading, cannot be carried out in practice because of the unfavorable viscosity behavior of uncrosslinked polyacrylic acids.
EP 391 012 describes an extrudable mixture for cable insulation which is based on butyl rubber and comprises a synthetic swelling powder based on polyacrylate. Sealing the gap between rubber sheath and outer conducting layer can be omitted with this mixture, however, it requires special machinery.
EP 397 410 describes the use of polyglycol semiesters of (meth) acrylic acids which, at first by incorporation through polymerization, and subsequently with thermal aftertreatment, can be cross-linked via the free alcohol group through an ester bond. This use has the decisive disadvantage that cross-linking periods of more than half an hour are required at temperatures of up to 200.degree. C., i.e., temperatures not harming the materials. This period is too long for an economically effective application.
EP 645 429 describes a water-miscible thickener for print pastes based on an oil-in-water emulsion. A mixture of carboxymethylcellulose and a copolymer of acrylic acid and acrylamido-2-methyl-propane sulfonic acid salt is used as polymer. Swelling properties of the paste so obtained are not disclosed.